Device for controlling a liquid flow in a tubular duct and particularly in a peristaltic pump

ABSTRACT

A device for sensing gas bubbles in a liquid flowing through a duct, particularly the outlet duct of a peristaltic pump, including two piezoelectric components shaped and excited so that they vibrate axially. One of the components is a transmitter and the other is a receiver. One of the piezoelectric components is secured to a movable support, and its displacement is detected in order to detect excess pressure in the liquid flowing through the duct.

The present invention relates to a device for monitoring the flow of aliquid in a tubular duct.

More precisely, the monitoring device serves at least to detect thepresence of bubbles of gas in the liquid flow, in particular when saidtubular duct is the tube of a peristaltic pump.

There exist certain situations in which it is important to make surethat a liquid flowing in a duct does not accidentally entrain bubbles,and in particular air bubbles. This is particularly true of the medicalfield for ducts which serve to convey perfusion or other liquid for apatient.

In certain circumstances, the liquid is conveyed towards the perfusionneedle in a duct, not merely under gravity, but with the help of a pump,e.g. a wheel or peristaltic pump. Such a pump is described, inparticular, in French patent application No. 2 691 258 filed in the nameof the Applicant. Such a disposition makes it possible to control moreaccurately the rate at which the liquid is injected. When using a pump,and regardless of the operating accuracy and leak-proofing it presents,it can always happen that there is an unacceptable quantity of air inthe liquid, coming from an accidental insertion of air into the tank,from too great a quantity of air dissolved in the liquid, or possiblyfrom an accidental cavitation effect causing bubbles of air to beentrained with the flowing liquid. It will naturally be understood thatsuch a situation is completely unacceptable and that it is particularlyimportant to be able to detect continuously any appearance of airbubbles in the duct, particularly at the outlet from the pump when sucha device is designed to be able to interrupt the admission of theliquid.

In order to detect the possible presence of gas or air bubbles,proposals have already been made for optical detection systems. That isdescribed, in particular, in the above-mentioned French patentapplication. Nevertheless, that optical detection can naturally only beused only when the liquid and the wall of the duct are translucent orsubstantially translucent.

To perform such detection, proposals have also been made to useultrasound, given that it is known that the transmission coefficient ortransmission impedance varies depending on the nature of the fluid, andthus varies depending on whether the liquid has no air bubbles or on thecontrary includes air bubbles. In known ultrasound detection systems, atransmitter and a receiver are used which are constituted bypiezoelectric cylinder disposed on either side of the tube, and thecylinders are excited in radial mode. That solution suffers from a firstdrawback which consists in the fact that, if the tube is placed in anenvironment that is itself liquid, the propagation of ultrasound due tothe poorly-directional transmission from the piezoelectric cylinder isdisturbed, thereby running the risk of allowing air bubbles to passwithout being detected. Another drawback of such ultrasoundtransmitter/receiver devices is that, in order to obtain an ultrasoundbeam that is relatively directional, it is necessary to use relativelyhigh excitation frequencies and consequently to perform detection onelectric signals that are likewise at high frequency. Typically, thesefrequencies are greater than one megahertz and generally lying in therange 2 MHz and 5 MHz. As a result, the electrical and electronicenvironment of such detectors is relatively complex and thus expensive.

Advantageously, the present invention provides a device for monitoringthe flow of a liquid in a duct, in particular for detecting bubbles ofgas in the liquid. This device is very reliable while being of low costand compact.

To achieve this aim, the device for monitoring the flow of a liquid in atubular duct is characterized in that it comprises:

a support structure;

a first piezoelectric cylinder whose vibration axis is substantiallyorthogonal to said duct in the detection zone and which is mounted onsaid support structure to be disposed on a first side of said duct;

means for exciting said first piezoelectric cylinder in axial mode at apredetermined frequency F in order to transmit ultrasound waves;

a second piezoelectric cylinder whose axis coincides substantially withthat of the first cylinder and mounted on said support structure so asto be disposed on a second side of said tube, whereby said secondcylinder receives said ultrasound waves after they have passed throughsaid duct;

means for picking up an electric signal representative of the amplitudeof axial vibration of said second cylinder; and

processor means for processing said electric signal to deduce therefromthe presence of a gas bubble, if any, in the liquid flowing in said ductthrough the detection zone.

It will be understood that by exciting the piezoelectric sensor in axialmode and by receiving the vibrations also in the axial excitation modeof the receiver, a highly directional ultrasound beam is obtained, andthe excitation frequency F can lie in the range 100 kHz to 1500 kHz,depending on the nature of the piezoelectric cylinder, which frequencyis very significantly lower than the previously known system.Consequently, the electric and electronic circuits can be considerablysimplified.

Preferably, the support structure is the support structure of aperistaltic pump, and the tubular duct is the outlet portion of thedeformable tube of said pump.

Preferably, in a plane perpendicular to the axis of the tube, thedimension of the transmitter/receiver face of the piezoelectric cylinderis no greater than the height of the bore of the tube.

Also preferably, the two piezoelectric cylinders are identical.

Also preferably, in the application of monitoring the liquid deliveredby a pump of the peristaltic type, the monitoring device simultaneouslyincludes means for detecting accidental excess pressure of the liquid inthe outlet tube, said excess pressure causing the tube to be deformed.

This disposition is particularly advantageous since it makes itpossible, if necessary, to stop operation of the pump in the event ofdifficulty in making the liquid flow, with such difficulty possiblybeing the result either of the tube being accidentally blocked, or elseof difficulty at the perfusion needle.

Other characteristics and advantages of the present invention appearbetter on reading the following description of various embodiments ofthe invention given as non-limiting examples. The description refers tothe accompanying figures, in which:

FIG. 1 is a simplified view of a device for detecting bubbles byultrasound;

FIG. 1A is a partial cross-sectional view of a cylinder shown in FIG. 1;

FIG. 2 is a vertical section view of a first embodiment of a device fordetecting bubbles applied to the case of a peristaltic pump, themonitoring device further including means for detecting excess pressurein the tube; and

FIG. 3 is a view analogous to FIG. 2 showing a second embodiment.

With reference initially to FIG. 1, an embodiment of the invention isdescribed making it possible to detect the presence of gas bubbles, ifany, in a liquid flowing in a duct.

In the figure, a tube 10 along which the liquid is flowing is shown inright section. The tube 10 is disposed in a support structure 12, 14that is shown diagrammatically. Two piezoelectric cylinders givenrespective references 16 and 18 are mounted on the support structure 12,14 on either side of the tube. The piezoelectric cylinders 16 and 18 areof the axial excitation type. They are generally cylindrical in shapehaving an actual length L that is greater than their diameter D.Electrodes 20 and 22 are formed at respective terminal ends of thetransmitting cylinder 16, and electrodes 24 and 26 are formed atrespective terminal ends of the receiving cylinder 18. The twopiezoelectric cylinders substantially share a common axis XX' lying inthe section plane of the duct 10 in the detection zone. In addition, theaxis XX' is substantially orthogonal to the axis of the duct 10. Theelectrodes 20 and 22 are respectively connected to afrequency-controlled AC signal generator 28 and to ground M. It will beunderstood that by applying excitation electrical pulses at apredetermined frequency F, preferably corresponding to the naturalfrequency of the piezoelectric cylinder, to the electrodes 20 and 22 bymeans of the pulse generator 28, the cylinder is caused to transmit adirectional ultrasound wave beam which passes through the duct 10 andthus through the liquid flowing therealong. The excitation frequency ofthe piezoelectric cylinder, when the cylinder is made of ceramics,preferably lies in the range 100 kHz to 1000 kHz; typically it is equalto 300 kHz. It will thus be understood that given its relatively lowfrequency, the pulse generator 28 and the associated electric circuitscan be of relatively standard structure.

The cylinders 16 and 18 are preferably identical in order to optimizecoupling.

Symmetrically, the receiving piezoelectric cylinder 18 converts itsfrequency of axial vibration into an electric signal at the samefrequency F, which electric signal is picked up by a detector 30. Suchdetectors are known per se. They are preferably synchronous with theexcitation frequency. By comparing the level of the received signal withpredetermined thresholds, the detector can detect the presence ofbubbles, if any, in the liquid flowing along the tube 10, because of amodification in acoustic impedance.

It should also be observed that given the excitation mode of thepiezoelectric cylinders 16 and 18, the ultrasound beam is highlydirectional and measurement is not disturbed even if the detectiondevice is itself in a liquid medium.

In order to obtain optimal detection of bubbles in the tube 10, in theplane of the figure, the dimension D of the piezoelectric cylinder 16 isno greater than the height H of the bore of the tube, and is preferablysubstantially equal thereto so as to detect all gas bubbles, if any.

For a tube having a height H of 3 mm, the ceramic cylinder 16 has adiameter of 2.5 mm and a length L of 5 mm. It is also possible to use astratified composite 17 (FIG. 1A) in the piezoelectric cylinder. Inwhich case, the right section of the cylinder is rectangular, e.g.having a height of 1.8 mm and a length (in a direction perpendicularlyto the plane of the figure) of 4 mm. Its thickness is 2.5 mm. The axialmode excitation frequency lies in the range 200 kHz to 1500 kHz, and ispreferably about 900 kHz.

After describing a simplified embodiment of the monitoring device fordetecting any gas bubbles in the liquid, there follows a descriptionwith reference to FIG. 2 of a first complete embodiment of themonitoring device applied to the case of a peristaltic pump, said deviceserving not only to detect the presence of any bubbles in the flowingliquid, but also to detect any excess pressure of the liquid in theduct.

FIG. 2 shows a portion of the support structure 50 of the peristalticpump, together with the outlet portion 52 of the flexible tube of theperistaltic pump. As already explained with reference to FIG. 1, thebubble detector device is constituted by two piezoelectric cylindersexcited in axial mode and referenced 54 and 56 respectively, which aredisposed on either side of the tube 52 and whose vibration axes YY' aresubstantially in alignment and orthogonal to the axis of the tube 52 inthe detection zone. The two cylinders are preferably identical, and ofone of the types described above. The front faces 54a and 56a of thesemiconductor cylinders are fixed by any appropriate means on respectivefront electrodes 58 and 60. As shown in FIG. 2, the electrodes definerespective cavities which preferably flare towards the tube. The activeface of each piezoelectric cylinder is fixed on the cavity-closing thinwall 58a, 60a of the electrode. This thin wall, interposed between theactive face of the cylinder and the duct 52 has a thickness e which isless than half the wavelength of the ultrasound waves transmitted by thepiezoelectric cylinder. For the above-described ceramic cylinder, thisthickness is typically equal to 0.3 mm. Also preferably, the outer faceof the electrode 56 is covered in a coating of the "Teflon" type inorder to provide insulation. Another solution consists in making thepieces 58 and 60 out of an electrically insulating material such asBakelite, with the electrode and the corresponding electrical conductorbeing inserted in the Bakelite. The electrodes 58 and 60 are pressedagainst the outer face of the wall 52a of the tube 52. More precisely,the electrode 58 associated with the piezoelectric cylinder 54 ismounted in fixed manner in the support structure 50 by means of asealing ring 62. The second electrode 64 of the cylinder 54 is fixed onits second end face and is connected to an electrical conductor 66. Thereceiver cylinder 56 is likewise naturally provided with a secondelectrode 68 fixed on its second end face and connected to anappropriate conductor 70. The set of piezoelectric cylinder 54 and 56serves to detect the presence of any bubbles in the liquid flowing inthe duct 52 by the process described with reference to FIG. 1.

In order also to detect any excess pressure of the liquid in the tube52, and thus any deformation of the flexible wall 52a of said tube, in apreferred embodiment of the invention the electrode 60 is extended by asleeve-forming piece 72 which surrounds the cylinder 56. The sleeve 72is slidably mounted in a cylindrical piece 74 secured to the supportstructure 50 of the peristaltic pump. A resilient flexible membrane 76provides a mechanical link between the moving electrode 60 and thesupport structure of the pump, thus allowing the electrode 60 to moverelative to the support structure under the effect of any expansion ofthe tube 52. In addition, the flexible membrane tends to urge the movingassembly and thus the piezoelectric cylinder 56 against the tube 52. Acap 78 closes the cavity 80 containing the piezoelectric cylinder 56 forsealing purposes.

To detect any increase in pressure, a pressure sensor 82, preferably ofthe resistive type, is interposed between a rigid fixed wall 84 of thesupport structure and a flexible piece 86, said flexible piece beingpressed against the cap 78, i.e. against the moving assembly containingthe cylinder 56. In this way any deformation of the tube 52 due toexcess pressure of the liquid is transmitted to the pressure sensor 82by the moving assembly 60, 78 and the deformable piece 86. The faces ofthe pieces 58 and 60 are preferably coated in a material having a verylow coefficient of friction, such a Teflon, as used for insulating thepiezoelectric cylinders. This avoids adding interfering stresses indirections other than those of the axis XX'. The sensor then delivers anelectric signal which, when processed by appropriate detector circuits,triggers an alarm signal or automatically causes operation of the pumpto stop.

FIG. 3 shows a variant embodiment of the detector device of FIG. 2. Inthis variant, the piezoelectric sensor 54 is mounted in analogous mannerto that of FIG. 2. It is therefore not described again. In contrast, thesensor 56 associated with the detector of excess pressure in the tube 52is mounted differently. This is described below.

The active face 56a of the second sensor 56 is fixed on the piece 100which is free relative to the housing 102 defined by the pieces 104 and106 secured to the main support structure 108. The piece 100 and thecylinder 56 are secured to the end 110 of a strain gauge 112 operatingin bending. The other end 114 of the gauge 112 is secured to the supportstructure 108, e.g. by being embedded therein. The strain gauge 112performs two functions. Firstly, by bending, it detects excess pressurein the tube of the pump, which excess pressure causes the piece 100 andthe sensor 56 to move and thus moves the end 110 of the sensor.Secondly, by its resilience, the gauge 112 tends to urge the piece 100continuously against the tube of the pump.

To seal the sensor 56, the housing 102 is filled with a gel which, onceset, is of a consistency suitable for enabling it to remain within thehousing 102, but which gives rise to no friction or other constraintswhen the piece 100 moves under the effect of excess pressure in the tubeof the pump.

To further improve the accuracy with which excess pressure in the tubeis detected, two pieces 120 and 122 disposed at a right-angle relativeto the axis of the sensors 54 and 56 are secured to the cassette. Thesepieces which have faces 120a and 122a in the form of portions of acylindrical surface, prevent the tube deforming in the orthogonaldirection ZZ', thus concentrating deformation of the tube in thedirection "X--X" of the axes of the sensors, and thus in thedisplacement direction of the end 110 of the strain gauge 112.

We claim:
 1. A monitoring device for monitoring the flow of a liquid ina tubular duct, comprising:a support structure of a peristaltic pumpincluding a deformable tube having an outlet portion forming the tubularduct; a first piezoelectric cylinder whose vibration axis issubstantially orthogonal to said duct in a detection zone and which ismounted on said support structure to be disposed on a first side of saidduct, said first cylinder being fixed relative to said supportstructure; means for exciting said first piezoelectric cylinder in anaxial mode at a predetermined frequency F lying in the range 100 kHz to1500 kHz, in order to transmit ultrasound waves; a second piezoelectriccylinder whose axis coincides substantially with that of the firstcylinder and that is disposed on a second side of said duct, wherebysaid second cylinder receives said ultrasound waves after they havepassed through said duct; said second piezoelectric cylinder beingsecured to a moving support moveable in translation relative to saidsupport structure along the direction of the axis of the cylinder; aresilient means interposed between said support structure and saidmoving support tending to press said second cylinder against the wall ofthe duct, a displacement detection means for detecting displacement ofsaid moving support relative to the support structure under the effectof deformation of said duct because of excess pressure of the liquidflowing along said duct; means for picking up an electric signalrepresentative of the amplitude of axial vibration of said secondcylinder; and processor means for processing said electric signal todeduce therefrom the presence of a gas bubble in the liquid flowing insaid duct through the detection zone.
 2. A device according to claim 1,characterized in that the displacement detection means comprise apressure sensor interposed between the support structure and one end ofsaid moving support.
 3. A device according to claim 2, characterized inthat said pressure sensor is a resistive sensor.
 4. A device accordingto claim 1, characterized in that said displacement detection meansincludes a strain gauge operating in bending having one end secured tosaid moving support and having its other end secured to the supportstructure, whereby said strain gauge tends to press said moving supportagainst the tubular duct of said pump, and detects displacements of saidsupport to detect any excess pressure in the tubular duct.
 5. A deviceaccording to claim 4, characterized in that said support and said secondpiezoelectric cylinder are mounted in a housing of the supportstructure, said housing being filled with a gel.
 6. A device accordingto claim 1, characterized in that said two cylinders are identical.
 7. Adevice according to claim 1, characterized in that, in a planeperpendicular to an axis of the duct, a dimension of atransmitter/receiver face of at least one of the piezoelectric cylinderis no greater than a height of a bore of said duct.
 8. A deviceaccording to claim 1, characterized in that said cylinders are made of aceramic optimal for said predetermined frequency.
 9. A device accordingto claim 1, characterized in that said cylinders are made of astratified composite material optimal for said predetermined frequency.10. A device according to claim 1, characterized in that at least one ofsaid piezoelectric cylinders includes a coating.
 11. A device accordingto claim 10, characterized in that said coating includes Teflon.